Tuesday, May 08, 2007

Exceptionally Active Weather for USA Probable Weeks 2-3

The spatial distribution of global tropical SSTs features warmth across the Indian Ocean-cool Indonesia-warm west central into South Pacific. Locations across the equatorial Pacific east of ~170W are again cooling while the Atlantic remains generally warm. Anomaly magnitudes are roughly 0.5-1C but near 2C just west of South America and portions of the equatorial Atlantic. SST totals remain ~30C across both the Indian and west Pacific basins and recently the east Pacific ~10N. Subsurface anomalies ~minus 3-4C remain along the equator around 130-140W at 100m within a cool reservoir down to ~200m from the west coast of South America to the date line. Development of La-Nina remains a monitoring issue but is appearing more likely.

Tropical convective forcing has become quite intense centered around 0/140E while extending in a west-northwest to east-southeast band from the Bay of Bengal into the South Pacific. This represents a fairly rapid eastward shift from a week ago. I attribute this change to strong complex interactions with the extratropics including a weak-moderate positive global and East Asian mountain torque during late April. This behavior is similar to what happened around April 1st, but much weaker. For instance, the global mountain torque around April 1st was ~plus 45 Hadleys while only 15 Haldeys recently. In any case, since about mid-February we have seen ~10-30 day variations involving global mountain torques, East Asia jet extensions and transient eastward shifts of tropical forcing across the Eastern Hemisphere. This has been in what we feel is a base state best described as GSDM Stage 1, and is seen from the quasi-phase space plot per link in the Appendix. Stated another way, the global wind has been oscillating (orbiting in phase space) around GSDM Stage 1, with the largest late March-late April. I see no reason for this end for at least weeks 2-3.

Zonal mean upper tropospheric westerly flow has increased slightly throughout the tropical and subtropical atmospheres, with anomalies ~2-5m/s. I expect this anomalous westerly flow will propagate to the midlatitudes and contribute to yet another intensification of the East Asian/North Pacific jet weeks 2-3. As already supported by full disk satellite and wind animation trends, tropical convective forcing should become more robust across the Indian Ocean weeks 2-3 while remaining active in other regions like the west Pacific. For the lower 48 states (my focus in these writings) this all suggests more strong troughs interacting with subtropical jets impacting the western USA ~forecast days 10-25.

The probable weather ramifications would be similar to what has already occurred several times since late February. A difference is for the general southwest flow storm track across the Plains to be farther northwest. Thus exceptional outbreaks of severe storms may again occur across the Plains perhaps starting by around next weekend.

Appendix

An experimental phase space plot of the GSDM utilizing normalized relative AAM tendency (Y-axis) and normalized relative AAM (X-axis) can be found at

These are probabilistic statements, and work is ongoing to quantify in future posts. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post an update this Friday while having to cover operational shifts.